Generalize Image support and add relative paths support for XWiki Syntax 2.1

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For images
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* Similar to links add the ability to add prefixes in a generic way. We have the following needs: 
* image where the source is at a URL 
* image where the source is at a relative path 
* image where the source is in a document

I propose the following syntax:

image:prefix:value

where prefix = one of http:, path:, attach:

Note that we already support image:http://... so the proposal here is simply to make it generic.

In addition I propose that when there's no prefix or if the prefix is unknown then to consider that the image is to be found in a document (equivalent to attach:).

Same pros as above.